This invention relates to methods of printing on thermographic paper.
Several different methods have been developed for the impactless printing of information on paper sheets or webs. In the well-known electrographic method, an electrostatic charged latent image is deposited on a dielectric paper and then developed by a liquid or solid toner. The toner may then, if necessary, be fused to the paper by heat or vapor.
In teledeltos printing a special paper is used which has a surface of a first color, usually white or light grey, and an undercoating of a contrasting color. The printing is accomplished by burning off portions of the surface of the paper in image configuration to expose the contrasting colored sublayer. This is usually accomplished by applying a voltage across the paper high enough to cause dielectric breakdown and therefore conduction in the surface layer.
U.S. Pat. No. 3,255,039 to H. R. Dalton describes an essentially similar method which uses a paper having an opaque porous coating over a contrasting color backing. A stylus having voltage applied thereto is used to selectively fuse the coating in the image area for making it transparent so that the undercoating becomes visible.
In practice all of these methods have substantial disadvantages which have prevented their widespread use in many applications. In electrographic printing, both the dielectric paper and the toner are relatively expensive and in teledeltos printing, the paper is both quite expensive and gives unsatisfactory image quality. In both of these methods extremely high voltages on the order of 1,000 volts are required for imaging the paper, which fact has further restricted their use.
Another system of impactless printing which has been considered recently is that of electrolytic printing. According to this method, the electrolysis of an electrolyte solution is used to form color. For example, a starch iodine reaction may be used in which potassium iodide is electrolyzed to obtain iodine which reacts with starch particles suspended in the electrolyte to form color. This system is attractive because starch and potassium iodide are inexpensive and because the printing may be performed on ordinary paper. However, a severe problem exists in that the image produced is not permanent and fades.
Attempts have been made to solve the fading problem by using more stable fade-resistant dyes or organic chemicals. A problem with this approach is that most organic materials are not very soluble in water and do not ionize easily. Even if the molecule has a high charge, the ionic mobility of a large molecule under the influence of an electric field is not as fast as that of an inorganic ion like iodine, and consequently a high printing speed may not be achieved.
Another common drawback with electrolytic printing is the short circuiting of electrodes due to either electrolytic deposition of metal on the electrodes or the formation of a salt bridge between electrodes.
In recent years there has been developed an impactless method of printing which alleviates many of these difficulties. According to this method, a special paper which develops color when heated to a temperature in the range of 70.degree. to 120.degree. centrigrade is used in conjunction with a heated writing stylus. The heating of the stylus is achieved by passing an electric current (normally a pulse) through a heating element at the stylus tip. U.S. Pat. No. 3,158,506 and 3,451,338 describe systems of this type.
Although the systems do not require high voltages and use a paper which is less expensive than that required in previous systems, they have not received as wide a use as they might. This is largely because of the fact that the stylus tip must go through a heating and cooling cycle for each dot or character printed, and thus the writing speed is limited to about 30 characters a second.